In the art of injection molding, machines are employed which cyclically supply plasticized material to a mold. These machines are usually of the reciprocating screw type in which the material to be molded is plasticized through the application of heat and the mechanical working of the material by the rotation of a screw within a plasticizing chamber. As the plasticized material is accumulated within the chamber, the screw, which also serves as a ram, retracts away from an injection orifice communicating with a mold cavity. When enough plasticized material has been accumulated, the ram advances toward the orifice in an injection stroke to inject the plasticized material into the mold.
One of the general problems in molding articles is that of insuring that the mold is filled properly with material. Because frequently the molds are quite intricate and irregular in shape, the material tends to flow through the mold in an erratic manner, first flowing into one region then another, and sometimes prematurely solidifying, blocking the flow to certain portions of the mold. This affects the surface finish of the objects and causes non-uniform density and irregular shrinkage of the objects. The rate at which the material flows through the passages of the molds will cause a change in the temperature and thus the viscosity of the material as it flows. By controlling this rate, the filling of the mold can be controlled to some degree. It has been found that injection of material into the molds at a precise programmed rate will greatly enhance the quality and uniformity of the molded products.
To achieve this, it has been attempted in some prior art systems to program the pressure exerted on the ram in order to achieve a more desirable flow pattern into the mold cavity. Some of these prior art devices have attempted to program this injection rate on a time basis. This has, however, not been entirely satisfactory, particularly in view of the fact that the flow rate into the mold is dependent on several variable factors, such as the viscosity of the material which is being injected and on various pressure fluctuations within the hydraulic ram driving system. Other systems have attempted to program this injection rate by mechanically synchronizing the position of the valve which supplies fluid to the ram directly to the position of the ram by mechanically actuating cam followers and switches. This again has not entirely overcome the injection rate problem in that intervening variables affect the relationship between the injection rate and the valve position.
It is one principal objective of the present invention to overcome the problems of the prior art and to more precisely control the flow rate of material into the mold in a precise programmed fashion.
Accordingly, the present invention as set forth in the parent application and the improvement set forth herein provides a means for programming the injection rate of material into the mold through the programming of the velocity of the ram. The present invention incorporates a velocity program module which operates to control the ram velocity as a function of the position of the ram in direct response to a closed loop feedback signal representative of the actual ram velocity.
The advantage of this particular aspect of the present invention is to overcome certain uncontrollable variables which affect the velocity of the ram such as the material viscosity and hydraulic system variables. Further advantages of this aspect of the invention are the ability to reduce "jetting", "blush" and warping of the product. For example, when mold filling starts too rapidly, material shoots into the empty mold and solidifies. This phenomenon is known as jetting and can be eliminated by the invention. Also, it has been found that surface stress is generally dependent on flow surface velocity, which is in turn dependent on a combination of the flow surface area of the material and the material flow rate. Irregular stresses can cause product warping. In accordance with the present invention, these irregular stresses and resultant warping are eliminated by precisely controlling ram velocity.
Another problem encountered by those systems of the prior art which have attempted to program the flow rate of material into the mold has been that the program has been inalterably tied in to the exact ram position. However, when the density and viscosity of the material vary, it is desirable to expand or contract the program or to alter the end points of the program in relation to the position of the ram so that the program need not be materially altered nor the mechanical linkages be moved on the machine.
It is another object of the present invention to provide a velocity program means which will provide velocity programming as a function of ram position but which will allow the program to be alterably associated with the actual position of the ram through simple adjustments within the programming module.
Accordingly, the present invention as set forth in this and the parent application provides a means which will associate a predetermined velocity control signal with a specific relative domain of the ram stroke and which will furthermore automatically divide the entire operable ram stroke into a plurality of distinct regions. The specific embodiment of the present invention will automatically divide a portion of the ram stroke between two arbitrary selectable end points and to associate the function directly to these regions. As these end points are moved for any reason, the present invention further provides that the program be automatically revised to redivide the new ram stroke domain into the same fixed number of regions and to associate the programmed velocity function with these corresponding regions. Since it may be necessary to revise the end points of this domain to accommodate for varying viscosity or density of the material to be molded, thereby lengthening or shortening the ram stroke, by the provision of the present invention, each portion of the ram velocity program will be directly related to the actual quantity of material fed as the stroke is varied to accommodate material density rather than in previous systems where the program was directly tied to fixed positions of the ram.
Another problem encountered in the prior art has been the difficulty in maintaining a predetermined flow rate into the mold while simultaneously insuring that the fill pressure of the mold cavity does not exceed certain critical values. If a critical value is exceeded, it is found that material will extrude from the junctures of the mold, resulting in what is referred to as a "flash". This not only reduces the actual material within the mold, but results in many cases in imperfect finish of the molded object and undesirable defect in the part.
It is a further objective of the present invention to provide means for insuring that the critical pressure of the mold is not exceeded when a precise injection rate program is employed.
Accordingly, the present invention provides means for monitoring the pressure within the mold cavity and utilizing this pressure to override the velocity program at the end of the injection stroke to limit the amount of material compressed in the mold to some desired value. Furthermore, the present invention provides for a closed loop feedback of the pressure signal from the mold cavity and the comparison of the signals of predetermined value and utilizing the result of this comparison in an analog fashion to override the velocity program. The overriding of the pressure in an analog fashion provides for more precise control of the servo valve which is supplying fluid to the ram, to regulate the deceleration of the ram while preventing loss of control and overshoot of the valve spool element, which may introduce an unpredictable effect on the final pressure within the mold.
Another major area wherein problems arise in the injection molding of objects is the difficulty in insuring that the objects produced fall within high dimensional and weight tolerances. It is important that objects be made in successive molding cycles of the machine in a highly predictable and repeatable manner.
One of the more critical problems in attaining precise repeatable articles of high dimensional and weight tolerances has been the phenomenon involving shrinkage of the molded article upon cooling. This shinkage is generally inversely related to the pressure and compressed density of the material within the mold at the time the mold is filled. In order to overcome these problems, some prior art attempts have been made to regulate the pressure within the mold at the time that it is filled. Another common practice in the art is to provide a cushion of material at the orifice of the extrusion device upon which a steady holding pressure is exerted so that material is forced into the mold to accommodate for the shrinkage of the material within the mold. However, as the mold cools, it becomes increasingly difficult to control the pressure within the mold cavity by the exertion of pressure by the ram against the cushion. One reason that this practice has not been wholly successful is that, as the viscosity of the material changes, the density of the material varies and thus the cushion size varies from cycle to cycle. Thus, the effect of the holding pressure operating through the cushion has differing effects from cycle to cycle upon the material within the mold cavity, and thus the density or weight and ultimate shrunk dimension of the molded products varies from cycle to cycle.
The factors which result in changes in viscosity and its effects on the molded material are discussed in detail in the copending application of the inventor of the subject matter of this application, filed Sept. 15, 1971, and entitled "Extruder Control System", now U.S. Pat. No. 3,759,648.
It is another important objective of the present invention to provide means for controlling the shrinkage of the objects molded from cycle to cycle in a precise and repeatable manner, and furthermore to control in a precise manner the quantity of material extruded to the mold in each cycle of operation. More particularly, it is an objective of the present invention to overcome the cycle to cycle effects of variable changes, such as the viscosity of the molded material.
Accordingly, the present invention, as set forth in this and the parent application, provides means of maintaining the cushion developed at the end of each injection stroke constant from cycle to cycle and furthermore provides additional means for insuring that the mold cavity fill pressure is also maintained constant from cycle to cycle. In addition, this constant pressure and constant cushion coexist at the same time in each cycle so that a precise pressure and volume relationship exists. This insures that a precise quantity of material, which is dependent on the combination of pressure and volume factors, is the same in each molding cycle. By maintaining the constant pressure in this manner, and by maintaining the constant cushion length through which a holding pressure applied by the ram is exerted, the cooling and consequent shrinkage characteristic of each product will be maintained in very close tolerances from cycle to cycle.
More particularly, the present invention provides a means for measuring the cushion length in each cycle of operation and for feeding this information in closed-loop feed-back manner to a control circuit which affects the cushion length during the next succeeding cycle of operation in a manner which will tend to maintain this cushion dimension constant from cycle to cycle. More particularly, the present invention provides means for measuring the cushion length at the precise instant that the cavity pressure has attained a predetermined value and for comparing this measured cushion dimension with a predetermined dimension. Furthermore, the present invention provides for utilizing the information derived in comparison of the actual and standard cushion dimensions to vary the shot size, or the retracted position of the ram at the beginning of the injection stroke, in the next injection cycle in a manner which will tend to correct for differences between the measured actual cushion dimension and the predetermined desired cushion dimension. Furthermore, the present invention provides means for setting a predetermined correction factor to a ram stroke so that, upon each comparison of the cushion dimension, the shot sizes vary by a predetermined fixed amount.
The automatic shot size correction capability which the present invention provides yields a particular advantage in allowing the injection molding machine to compensate for slowly varying changes in material density and viscosity and also provides means to automatically correct for any improper setting of the shot size by the operator and to allow for only a rough initial setting which will be automatically followed by the adapting of the machine to the optimum shot size for the given product being molded.
Furthermore, the effect of the present invention is to provide precise control, not only of pressure, but volume and temperature at the time of mold filling. By this provision, it is possible with the present invention to precisely control part size by adjustment of cavity pressure. This has not been provided before by any system of the prior art since, because of other uncontrollable variables, no prior art system has provided the precise relationship between part size and cavity size.
Furthermore, an additional objective of the present invention is to provide means which are economical and efficient to adapt an injection molding machine to complete computerized control.
Accordingly, the present invention provides a programming module which may serve as an interface between a conventional injection molding machine and a computer. To achieve this, the present programming module of the present invention undertakes to furnish and control those aspects of an injection machine operation which are peculiar to the injection molding process and the particular machine being used and, in addition, the particular molded object which is being used and formed. In this manner, sophisticated computer master process controls may be used without the necessity of programming these computers to the particular characteristics and properties of different injection molding machines.
Another problem encountered in injection molding is the phenomenon known as blush. This occurs when the cavity pressure is released too rapidly while the material is still molten. In many cases it is desirable to fill a cavity to a relatively high cavity pressure and then to relieve to a somewhat reduced holding pressure which is sustained until the material solidifies. In lowering the pressure to the holding pressure, the ram will normally retract some finite dimension. If the pressure is dropped too rapidly, the ram will tend to overshoot in this retracted position.
It is one of the objectives of the present invention, and particularly of the improvement disclosed herein, to alleviate this problem by controllably decreasing from the cavity pressure to the holding pressure along a ramp of limited slope.
Accordingly, the improvement of the present invention provides means for switching the ram pressure from the relatively high preset cavity pressure at the end of injection to the relatively low holding pressure in a manner which provides a smooth and gradual ram pressure decrease during the transition between injection and hold. In the preferred embodiment, this is accomplished by means of servomotor-controlled flow divider valve of the shiftable spool type which, in a controllable manner dependent on spool position, divides the flow from a pump between the ram pressure chamber and a drain tank. Since spool position, which is accurately controlled by a servomotor, establishes the operating pressure, and any positional shift thereof to alter pressure is reasonably smooth, change in pressure from one level to another during transition from injection to hold occurs in a gradual manner, avoiding problems associated with ram overshoot occasioned by a sudden pressure drop.
A further advantage of using a flow divider between the ram pressure cylinder and the drain tank, particularly in a system where ram velocity is programmed, is that the response of the system to a sudden resistance to ram motion, and hence decrease in ram velocity below the programmed level, is significantly improved over prior systems. More particularly, in this invention when a decrease in ram velocity below the programmed level is sensed and the flow divider spool shifted, to increase fluid flow to the ram, there is a simultaneous and complementary increase in blockage of the flow path to the drain tank. As a consequence, the pressure of the hydraulic fluid increases concurrently with the increase in flow rate to the ram, assuring that the increased flow to the ram will occur at a pressure sufficient to overcome the increased resistance to ram motion which initially caused the velocity drop, thereby restoring the ram velocity to the programmed level without undue sluggishness typical of velocity control schemes heretofore used.
A futher object of the present invention is to provide means which more precisely regulate the ram fluid pressure throughout the entire molding cycle. Particularly, the present invention involves using the flow divider valve, which is connected to the ram cylinder, in a closed-loop servo system during all portions of the cycle, i.e., during injection, hold and plasticize. This single flow divider valve replaces the multiplicity of valves previously required. The hydraulic equipment, as disclosed in the parent application, normally included separate valves for setting the holding and plasticizing pressure. By direct servo-control of the flow divider valve and provision of electronic means for generating the proper servo signals during all phases of the molding cycle, the improvement of the present invention requires only a single valve to control the various portions of the machine cycle. In addition, it provides a great conservation of power in that unused fluid from the high pressure pump which drives the system is bypassed by the flow divider at low pressure back to the fluid reservoir, thus expending a minimum amount of energy.
A further object of the present invention is to provide a means for switching from cavity pressure control to fluid pressure control. In the first instance, cavity pressure is utilized as a feed-back control to terminate the injection stroke portion of the cycle, while fluid pressure control is used during the holding and plasticizing portions of the cycle. Separate transducers are provided in the molding cavity and in the fluid lines to generate these respective feed-back signals for the servo-control. This enables cavity pressure and ram pressure during hold and plasticization, critical parameters in an injection molding process, to be directly monitored by the mold cavity and ram pressure cylinder transducers, respectively, in turn permitting control of charge size during injection in direct response to actual cavity pressure, and control of the ram during hold and plasticization in direct response to actual ram cylinder pressure.
A further object of the present invention is the provision of means for controlling the back pressure and/or screw speed in a programmed manner as a function of ram position or time during plasticization as the ram is being retracted. This aspect is provided through the use of a separate patch panel which takes over control of the operation during the plasticizing stroke. By programming screw speed and/or back pressure during plasticization, a predetermined temperature profile can be imparted to the charge along the length thereof while in the barrel prior to injection. This enables controlled variation in density of the molded article throughout its volume to achieve desired variable levels in preselected characteristics such as surface wear, gloss, resolution and the like.
These and other objectives and advantages of the present invention will be more readily apparent from the following detailed description of the drawings illustrating a preferred embodiment of the injection molding control system of the present invention, and modifications thereof, in a reciprocating screw type injection molding machine.